High strength cold rolled steel sheet and method for manufacturing the same

ABSTRACT

This disclosure relates to a high strength cold rolled steel sheet composed of ferrite grains having an average grain diameter of 10 μm or less, in which the average number per unit area of Nb(C, N) precipitates having a diameter of 50 nm or more is 7.0×10 −2 /μm 2  or less, and a zone having a width of 0.2 to 2.4 μm and an average area density of NbC precipitates of 60% or less of that of the central portion of the ferrite grains is formed along grain boundaries of the ferrite grains, for example, the steel sheet consisting of 0.004 to 0.02% of C, 1.5% or less of Si, 3% or less of Mn, 0.15% or less of P, 0.02% or less of S, 0.1 to 1.5% of sol.Al, 0.001 to 0.007% of N, 0.03 to 0.2% of Nb, by mass, and the balance of Fe and inevitable impurities. The steel sheet is most preferably used for automobile panel parts since it has the TS of 340 MPa or more and the superior surface strain resistance and press formability.

TECHNICAL FIELD

This disclosure relates to a high strength cold rolled steel sheet usedfor automobiles, home appliances, or the like, in particular, to a highstrength cold rolled steel sheet having superior press formability and atensile strength TS of 340 MPa or more, and to a manufacturing methodthereof.

BACKGROUND ART

Heretofore, for automobile panel parts having a complicated shape suchas a side panel or a door inner panel, which are difficult to be pressformed, interstitial free (IF) cold rolled steel sheets (270E, F) havingsuperior deep drawability and stretchability and a TS of around 270 MPa,have been widely used.

In recent years, due to increasing needs of lighter weight and higherstrength of automobile bodies, a high strength cold rolled steel sheethaving a TS of 340 MPa or more, and particularly, 390 MPa or more, hasbeen progressively applied to those parts which are difficult to bepress formed. In addition, as is the case described above, there hasalso been a trend to apply a higher strength cold rolled steel sheet toinner parts or the like, in which a high strength cold rolled steelsheet has been used, so as to further reduce automobile weight bydecreasing the number of reinforcement parts or by decreasing thethickness thereof.

However, when the strength of the high strength cold rolled steel sheetused in automobile panels is further increased, and the thicknessthereof is further decreased, the occurrence of surface strain isremarkably increased due to the increase in yield strength YS, thedecrease in work hardening coefficient n value, and the decrease in thethickness. This surface strain is a defect such as an undulation or awrinkle brought out on a surface of steel sheet after press forming anddeteriorates dimensional precision or appearance of press formed panels.Therefore, when a high strength cold rolled steel sheet is applied toparts which are difficult to be press formed such as automobile panelparts, the steel sheet must have superior resistance to surface strainand excellent stretchability, and more particularly, the steel sheethaving a YS of 270 MPa or less and a n₁₋₁₀ of 0.20 or more is preferablydesired. Here, the n₁₋₁₀ is a work hardening coefficient calculated fromthe stresses at strains of 1% and 10% of a stress-strain curve obtainedfrom a tensile test.

To decrease the yield ratio YR (=YS/TS), a method has been well known,in which a Ti or Nb added steel having the amount of C and N decreasedas small as possible is hot rolled and coiled at a temperature of 680°C. or more to decrease the number of precipitates containing Ti or Nband thereby to promote grain growth at annealing after cold rolling. Inaddition, in Japanese Unexamined Patent Application Publication No.6-108155 and Japanese Patent No. 3291639, methods for promoting graingrowth have been disclosed in which the amounts of C and S of Ti addedsteel are controlled to bring about Ti(C, S) precipitates in order tosuppress the formation of fine TiC precipitates.

The above-mentioned methods are effective for a cold rolled mild steelsheet having a TS of approximately 270 MPa. However, when the graingrowth is promoted, the TS is also decreased simultaneously as the YS isdecreased, and therefore the methods are not always effective for a highstrength cold rolled steel sheet having a TS of 340 MPa or more. Thatis, since the decrease in TS must be compensated for by addition ofalloying elements such as Si, Mn, or P, problems may arise in that amanufacturing cost is increased, surface defects take place, a YS of 270MPa or less is not obtained, and the like. For example, when the steelsheet is strengthened by addition of Si, Mn, and P, accompanied by thegrain growth of approximately 10 μm to 20 μm in grain size, the steelsheet can only be obtained having a YS approximately 10 MPa smaller thanthat of a conventional high strength cold rolled steel sheet, and inaddition, the resistance to the occurrence of orange peel and theanti-secondary work embrittlement of the steel sheet also deteriorates.

On the other hand, in Japanese Unexamined Patent Application PublicationNos. 2001-131681, 2002-12943, and 2002-12946, methods have beendisclosed in which, without promoting grain growth, the YS is decreasedand the high n value is obtained. According to the methods describedabove, the amount of C is controlled to approximately 0.004 to 0.02%,which is larger than that of a conventional ultra low carbon steelsheet, and grain refinement and precipitation strengthenings arepositively applied in order to decrease the YS by approximately 20 MPathan that of a conventional ultra low carbon IF steel sheet.

However, when a high strength cold rolled steel sheet having a TS ofapproximately 390 MPa or 440 MPa is manufactured by the methodsdescribed above, the YS exceeds 270 MPa, and it becomes difficult toperfectly suppress the occurrence of the surface strain.

It could therefore be advantageous to provide a high strength coldrolled steel sheet having a TS of 340 MPa or more, in which YS≦270 MPaand n₁₋₁₀≧0.20 are satisfied, and a manufacturing method thereof, thesteel sheet having superior surface strain resistance and pressformability.

SUMMARY

We thus provide a high strength cold rolled steel sheet composed offerrite grains having an average grain diameter of 10 μm or less, inwhich the average number per unit area (hereinafter referred to as“average area density”) of Nb(C, N) precipitates having a diameter of 50nm or more in the ferrite grains is 7.0×10⁻²/μm² or less, and a zone(hereinafter referred to as “PFZ”) having a width of 0.2 to 2.4 μm andan average area density of NbC precipitates of 60% or less of that ofthe central portion of the ferrite grains is formed along grainboundaries of the ferrite grains.

This high strength cold rolled steel sheet can be obtained, for example,by a high strength cold rolled steel sheet consisting of 0.004 to 0.02%of C, 1.5% or less of Si, 3% or less of Mn, 0.15% or less of P, 0.02% orless of S, 0.1 to 1.5% of sol.Al, 0.001 to 0.007% of N, 0.03 to 0.2% ofNb, by mass, and the balance of Fe and inevitable impurities.

In addition, this high strength cold rolled steel sheet can bemanufactured by a manufacturing method comprising the steps of: hotrolling a steel slab having the composition described above into a hotrolled steel sheet after heating the steel slab at a heating temperatureSRT which satisfies the following equations (3) and (4); and picklingand cold rolling the hot rolled steel sheet, followed by annealingwithin a temperature range of a ferrite phase above therecrystallization temperature.SRT≦1350° C.  (3), and1050° C.≦SRT≦{770+([sol.Al]−0.085)^(0.24)×820}° C.  (4),where [sol.Al] represents the amount of sol.Al (mass %).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the relationship between amount of sol.Al and YS, n valueand r value.

FIG. 2 shows the relationship between amount of sol.Al and slab heatingtemperature and YS.

DETAILED DESCRIPTION 1. Control of Precipitates Containing Nb

We investigated how to decrease the YS of a high strength cold rolledsteel sheet and clarified that a high strength cold rolled steel sheethaving a YS of 270 MPa or less, an n₁₋₁₀ of 0.20 or more, and a TS of340 MPa or more can be obtained when the steel sheet is composed offerrite grains having an average grain diameter of 10 μm or less, inwhich the average area density of Nb(C, N) precipitates having adiameter of 50 nm or more is controlled to 7.0×10⁻²/μm² less, and a zonehaving a width of 0.2 to 2.4 μm and an average area density of NbCprecipitates of 60% or less of that of the central portion of theferrite grains is formed along grain boundaries of the ferrite grains.

The Nb(C, N) precipitates having a diameter of 50 nm or more are formedat hot rolling to have a diameter of approximately 50 nm, do not becomelarger even at annealing after cold rolling, and are uniformly dispersedin the ferrite grains.

On the other hand, the NbC precipitates at the center of the ferritegrains are formed at annealing, the diameter of which is approximately10 nm, and the NbC precipitates in the PFZ are formed in such a way thatfine precipitates having a diameter of approximately 2 nm uniformlyformed at hot rolling are coarsened to have a diameter of approximately50 nm by Ostwald-ripening.

The average area density of NbC and Nb(C, N) precipitates was measuredas described below using a transmission electron microscope at amagnification of 5610 times and an accelerating voltage of 300 kV.

As to the Nb(C, N) precipitates having a diameter of 50 nm or moreuniformly formed in the ferrite grains, arbitrary 50 portions thereinwere selected, the number of Nb(C, N) precipitates existing in a circleof 2 μm in diameter centered at each of the portions was measured tocalculate the number per unit area (area density), and finally theaverage was obtained therefrom.

The average area density of NbC precipitates in the central portion ofthe ferrite grains was obtained in the same manner as described above.

As to the NbC precipitates in the PFZ, arbitrary 50 precipitatescoarsened by Ostwald-ripening were selected. For each of the NbCprecipitates, a circle inscribed with the NbC and the grain boundaryadjacent to the NbC was described, the number of NbC precipitatesexisting in the circle was measured to obtain the area density, and theaverage of the area density was then calculated.

The width of the PFZ was obtained as the average of the diameters of theabove 50 circles.

The high strength cold rolled steel sheet has the central portion offerrite grain in which fine NbC precipitates having the diameter ofapproximately 10 nm are formed at a high density and the PFZ along thegrain boundary in which coarse NbC precipitates having the diameter ofapproximately 50 nm are formed at a low density. It is considered that alow YS and a high n value can be obtained because the soft PFZ isdeformed by a low stress at the initial stage of the plasticdeformation, and that a high TS can be obtained due to the hard centralportion of ferrite grain.

As previously mentioned, the fine NbC precipitates having a diameter ofapproximately 2 nm are uniformly formed at the hot rolling and coarseninto the precipitates having the diameter of approximately 50 nm on thegrain boundary of recrystallized ferrite grains at annealing in acontinuous annealing line (CAL) or a continuous galvanizing line (CGL)after cold rolling. Therefore, the PFZ is believed to be formed due topromotion of grain boundary migration.

In order not to make ferrite grains extremely coarse, the recrystallizedgrains should be preferably as fine as possible, and the PFZ can be moreeffectively formed.

2. Chemical Composition

As a high strength cold rolled steel sheet, for example, there may bementioned a cold rolled steel sheet consisting of 0.004 to 0.02% of C,1.5% or less of Si, 3% or less of Mn, 0.15% or less of P, 0.02% or lessof 5, 0.1 to 1.5% of sol.Al, 0.001 to 0.007% of N, 0.03 to 0.2% of Nb,by mass, and the balance of Fe and inevitable impurities. C, Nb, andsol.Al play a very important role in the control of NbC and Nb(C, N)precipitates, and the amounts of C, Nb, and sol.Al must be controlled asfollows.

C: Since C is combined with Nb, C plays an important role in the controlof NbC and Nb(C, N) precipitates. The amount of C is set to 0.004 to0.02%, preferably 0.004 to 0.01%.

Nb: In order to control the NbC and Nb(C, N) precipitates, the amount ofNb is set to 0.03% or more. However, when the amount of Nb exceeds 0.2%,the increase in the rolling load at the hot rolling and the cold rollingcauses the decrease in productivity or the increase in cost. Therefore,the amount of Nb is set to 0.2% or less.

In order to increase r value, ([Nb]/[C])×(12/93)≧1 is preferablysatisfied, and the ([Nb]/[C])×(12/93) is more preferably 1.5 to 3.0.

sol.Al: Even when the amount of C is controlled to 0.004 to 0.02%, andthe amount of Nb is controlled to 0.03 to 0.2%, Ys of 270 MPa or lessmay not always be obtained in some cases. It is considered to be due tocoarse Nb(C, N) precipitates formed at hot rolling. As theabove-mentioned, it is believed that the coarse Nb(C, N) precipitateshaving the diameter of approximately 50 nm which is formed at the hotrolling have difficulties to be coarsened by Ostwald-ripening atannealing because of the large size and the smaller solubility inferrite than that of NbC precipitates, and suppression of the PFZformation leads to suppression of the decrease in YS.

Then, we investigated a method for the formation of NbC precipitateseffective for forming the PFZ by suppressing coarse Nb(C, N)precipitates having a diameter of 50 nm or more, and found that theaddition of 0.1% or more of sol.Al is effective.

It has been believed that N is combined with Al to form AlN. However, insteel containing 0.004% or more of C and 0.03% or more of Nb,precipitation of Nb(C, N) takes place at finish rolling before AlNstarts to precipitate. When the amount of Al is increased to 0.1% ormore so that AlN is precipitated before Nb(C, N) is precipitated, theprecipitation of NbC effective for forming the PFZ can proceed.

FIG. 1 shows the relationship between the amount of sol.Al and YS, nvalue and r value.

The results shown in FIG. 1 were obtained by investigating YS, r value,and n value of cold rolled steel sheets containing 0.0060% of C, 0 to0.45% of Si, 1.5 to 2% of Mn, 0.02% of P, 0.002% of S, 0.003% of N,0.0005% of B, 0.11% of Nb, and 0.01 to 1.7% of sol.Al, which are heatedat 1150° C. and 1250° C., followed by the hot rolling to 3 mm thick inthe γ region and coiling at 560° C., and subsequently cold rolled to 0.8mm thick, followed by annealing at 820° C. for 80 seconds. Since theincreases in TS by the addition of one percent of Si, Mn, and sol.Alwere 86 MPa, 33 MPa, and 32.5 MPa, respectively, the amounts of Si, Mn,and Al were controlled so as to obtain a constant TS of approximately440 MPa. That is, ([Si]+[Mn]/2.6+[sol.Al]/2.6) was controlled to 1.25%.Here, [M] represents the amount of element M (mass %).

YS, r value, and n value are also examined in a conventional ultra lowcarbon cold rolled steel sheet manufactured under the same conditions asdescribed above using a steel containing 0.0020% of C, 0.75% of Si, 2%of Mn, 0.02% of P, 0.002% of S, 0.003% of N, 0.0005% of B, 0.015% of Nb,and 0.03% of Ti.

The cold rolled steel sheets containing 0.004% or more of C and 0.03% ormore of Nb have lower YS, higher n value, and higher r values than theconventional ultra low carbon cold rolled steel sheet. In particular,when the amount of sol.Al is 0.1 to 1.5%, YS becomes 270 MPa or less andn₁₋₁₀ becomes 0.20 or more. In addition, when the amount of sol.Al is0.2 to 0.6%, the YS is further decreased to 260 MPa or less in bothcases of heating temperatures of 1250 and 1150° C. The ferrite grainswere sufficiently fine as is the case in which the amount of sol.Al is0.1% or less.

When the amount of sol.Al is less than 0.1%, a large number of Nb(C, N)precipitates having a diameter of 50 nm or more, which inhibit theformation of the PFZ, are observed. On the other hand, when the amountof sol.Al is 0.1 to 1.5%, the coarse Nb(C, N) precipitates areremarkably decreased to an average area density of 0 to 7.0×10⁻²/μm²,and the PFZ is remarkably formed.

The reason why the r value is remarkably increased when the amount ofsol.Al is controlled to 0.1% or more is not clear. It is, however,inferred that Al influences the formation of a deformation band at coldrolling or on the amount of solute C.

Si: Si is an element for the solid solution strengthening, which may beadded when it is necessary. However, the amount of Si which exceeds 1.5%deteriorates the ductility and the anti-secondary work embrittlement,and increases the YS. The amount of Si is set to 1.5% or less. Inaddition, since the addition of Si deteriorates the conversion treatmentproperties of a cold rolled steel sheet and appearance of a hot dipgalvanized steel sheet, the amount of Si is preferably set to 0.5% orless. In order to strengthen the steel sheet, the amount of Si ispreferably set to 0.003% or more.

Mn: Since Mn is also an element for solid solution strengthening and anelement for preventing red shortness, Mn may be added when it isnecessary. However, when the amount of Mn exceeds 3% a decrease inductility and an increase in YS occur. The amount of Mn is set to 3% orless. To obtain the superior appearance of the galvanized steel sheet,the amount of Mn is preferably set to 2% or less. The amount of Mn ispreferably set to 0.1% or more for the solid solution strengthening.

P: P is an effective element for strengthening the steel. However,excessive addition of P deteriorates the anti-secondary workembrittlement and ductility, and causes an increase in YS. Therefore,the amount of P is set to 0.15% or less. To prevent the deterioration ofalloying treatment properties and adhesion failure of coating of thegalvanized steel sheet, the amount of P is preferably set to 0.1% orless. The amount of P is preferably set to 0.01% or more to increase thestrength of the steel sheet.

S: S exists as a sulfide in the steel sheet. Since an excessive amountof S decreases ductility, the amount of S is set to 0.02% or less.0.004% or more of S is desirable for the descaling preferably set to,and 0.01% or less of S is favorable for the ductility.

N: Since N is necessary to precipitate as AlN with the addition of 0.1to 1.5% of sol.Al, the amount of N is set to 0.007% or less. The amountof N is preferably decreased to as little as possible. However, sincethe amount of N can not be decreased to less than 0.001% by the steelsmelting process, the amount of N is set to 0.001% or more.

The balance is Fe and inevitable impurities.

In addition to the elements described above, at least one elementselected from the group consisting of 0.0001 to 0.003% of B, 0.5% orless of Cu, 0.5% or less of Ni, 0.3% or less of Mo, 0.5% or less of Cr,0.04% or less of Ti, 0.2% or less of Sb, and 0.2% or less of Sn ispreferably added for the following reasons.

B: The amount of B is set to 0.0001% or more in order to improve theanti-secondary embrittlement. When the amount of B exceeds 0.003%, theeffect saturates, and the rolling load at hot rolling is increased.Therefore, the amount of B is set to 0.0001 to 0.003%.

Cu, Ni, Mo, and Cr: In order to increase the TS, the anti-secondary workembrittlement, and the r value, 0.5% or less of Cu, 0.5% or less of Ni,0.3% or less of Mo, and 0.5% or less of Cr may be added. Cu, Cr, and Niare the expensive elements, and when the amount of each element exceeds0.5%, the surface appearance deteriorates. Although Mo increases the TSwithout decreasing the anti-secondary work embrittlement, the amount ofMo exceeding 0.3% increases the YS. When Cu, Cr, and Ni are added, theamount of each element is preferably set to 0.03% or more. When Mo isadded, the amount of Mo is desirably set to 0.05% or more. When Cu isadded, Ni is preferably added with the same amount as Cu.

Ti: To improve the r value, 0.04% or less of Ti may be added. The amountof Ti exceeding 0.04% increases coarse precipitates containing Ti, whichlead to a decrease in the TS and prevention of a decrease in the YS bysuppressing AlN precipitation. When Ti is added, the amount of Ti ispreferably set to 0.005% or more.

Sb and Sn: To improve the surface appearance, the coating adhesion, thefatigue resistance, and the toughness of the galvanized steel sheet,0.2% or less of Sb and 0.2% or less of Sn are effectively added so that0.002≦[Sb]+½×[Sn]≦0.2 is satisfied. [Sb] and [Sn] represent the amountsof Sb and Sn (mass %), respectively. Since the addition of Sb and Snprevents surface nitridation or oxidation at slab heating, at coilingafter hot rolling, at annealing in a CAL or a CGL, or at additionalintermediate annealing, the coating adhesion is improved in addition tothe suppression of the irregular coating. Furthermore, since adhesion ofzinc oxides to the steel sheet in a coating bath can be prevented, thesurface appearance of the galvanized steel sheet is also improved. Whenthe amounts of Sb and Sn exceed 0.2%, they deteriorate the coatingadhesion and the toughness of the galvanized steel sheet.

3. Manufacturing Method

The high strength cold rolled steel sheet can be manufactured by amanufacturing method comprising the steps of: hot rolling a steel slabhaving a selected chemical composition into a hot rolled steel sheetafter heating the steel slab at a heating temperature SRT whichsatisfies the following equations (3) and (4); and pickling and coldrolling the hot rolled steel sheet, followed by annealing within atemperature range of a ferrite phase above the recrystallizationtemperature.SRT≦1350° C.  (3), and1050° C.≦SRT≦{770+([sol.Al]−0.085)^(0.24)×820}° C.  (4),where [sol.Al] represents the amount of sol.Al (mass %).

As shown in FIG. 1, when the amount of sol.Al is 0.1 to 0.6%, the lowerYS can be obtained at the heating temperature SRT of 1150° C. ascompared with that of 1250° C.

The relation between the amount of sol.Al and SRT and YS wasinvestigated by using the cold rolled steel sheets shown in FIG. 1.

As shown in FIG. 2, when the amount of sol.Al is 0.1 to 0.6%, andSRT≦{770+([sol.Al]−0.085)^(0.24)×820}° C. is satisfied, the low YS suchas 260 MPa or less can be obtained. It is believed to be caused by thesuppression of Nb(C, N) precipitation at hot rolling, accompanied by thesuppression of AlN dissolution at heating by controlling the SRT. Fineferrite grains having a grain diameter of 10 μm or less were obtained.

To obtain the superior surface quality, the scales formed at slabheating and at hot rolling should be preferably sufficiently removed.Heating with a bar heater at hot rolling may also be performed.

The coiling temperature after hot rolling influences formation of thePFZ and the r value. To effectively form the PFZ, fine NbC must beprecipitated, and to obtain a high r value, the amount of solute C mustbe sufficiently decreased. In view of the effective formation of the PFZand the high r value, the coiling temperature is preferably set to 480to 700 C.°, more preferably 500 to 600 C.°.

The coiling temperature after hot rolling has influences on theformation of PFZ and the r value. In order to effectively form the PFZ,fine NbC must be precipitated, and in order to obtain a high r value,the amount of solute C must be sufficiently decreased. In view of theeffective formation of PFZ and the high r value, the coiling temperatureis preferably set to 480 to 700° C., more preferably 500 to 600° C.

The high cold rolling reduction is desirable. However, a cold rollingreduction which exceeds 85% increases the rolling load, so that theproductivity decreases. Therefore, the cold rolling reduction ispreferably 85% or less.

A high annealing temperature promotes the precipitation of coarser NbCexisting in the vicinity of grain boundary, which causes the low YS andthe high n value. Therefore, the annealing temperature is preferably setto 820 C.° or more. When the annealing temperature is lower than therecrystallization temperature, the sufficiently low YS and the high nvalue can not be obtained. Therefore, the annealing temperature must beat least not less than the recrystallization temperature. However, whenthe annealing temperature exceeds the Ac1 transformation temperature,ferrite grains become very fine by the ferrite transformation from theaustenite, which leads to increase the YR. Therefore, the annealingtemperature must be the temperature of the Ac1 transformationtemperature or less.

When the annealing time is increased, grain boundary migration occursmore significantly to promote the formation of the PFZ. Therefore, thesoaking time is preferably set to 40 seconds or more.

A cold rolled steel sheet after annealing may be galvanized byelectrogalvanizing or hot dip galvanizing. The excellent pressformability can also be obtained in the galvanized steel sheet wherepure zinc coating, alloy zinc coating, and zinc-nickel alloy coating maybe applied. Even when the organic film is deposited after the coating,the superior press can also be obtained.

Example 1

Several types of steel A to V having the chemical compositions listed inTable 1 were smelt and continuously cast into the slabs having athickness of 230 mm. These slabs were heated to 1090 to 1325° C. and hotrolled to 3.2 mm thick under the hot rolling conditions listed in Table2. These hot rolled steel sheets were cold rolled to 0.8 mm thick,followed by annealing in a continuous annealing line (CAL), a hot dipgalvanizing line (CGL), or a box annealing furnace (BAF) under theannealing conditions shown in Table 2, and subsequently, temper rollingwith the elongation of 0.5%.

The hot dip zinc coating was performed at 460° C. in the CGL, followedby the alloying treatment of the coated layer at 500° C. in an in-linealloying furnace. The amount of the coating per one surface was 45 g/m².

The tensile tests were performed using JIS No. 5 test pieces cut fromthe direction of 0°, the direction of 45° and the direction of 90° tothe rolling direction, respectively. The averages of YS, n₁₋₁₀, r value,and TS were obtained by the following equation, respectively. Theaverage V=([V0]+2[V45]+[V90])/4, where [V0], [V45] and [V90] show thevalue of the properties obtained in the direction of 0°, 45° and 90° tothe rolling direction, respectively.

The ferrite grain diameter was measured by the point-counting method inthe rolling direction, the thickness direction, and the direction of 45°to the rolling direction at the cross section parallel to the rollingdirection, and the average of the ferrite grain sizes was obtained. Thesizes of NbC and Nb(C, N) and the average area density thereof wereobtained by the method previously mentioned.

The results are shown in Table 2.

Samples Nos. 1 to 19 have the YS of 270 MPa or less, the n₁₋₁₀ of 0.20or more, and the high r value of 1.8 or more. In particular, the samplesNos. 2 to 6, 9 to 11, 15 to 17, and 19 have the YS of 260 MPa or lessbecause the amounts of sol.Al are 0.1 to 0.6% and the temperature arewithin this disclosure. In the case of our samples, the average areadensity of coarse Nb(C, N) precipitates having a diameter of 50 nm ormore, which prevents the formation of the PFZ, is 7.0×10⁻²/μm² or less,and the PFZ having a width of 0.2 to 2.4 μm was formed in the vicinityof the ferrite grain boundary.

On the other hand, samples Nos. 20 to 27 of the comparative exampleshave the high YS and the low n value because the average area density ofcoarse Nb(C, N) precipitates having a diameter of 50 nm or more or thewidth of the PFZ is out of this disclosure. Sample No. 20 in which theamount of sol.Al is small has the YS of more than 270 MPa, then value ofless than 0.20, and the r value of less than 1.8. Sample No. 21 in whichthe amount of sol.Al is excessive has the YS of more than 270 MPa andthe n value of less than 0.20. Samples Nos. 23, 24, 25, and 26 in whichC, Si, Mn, and P are out of our range have the YS of excessively largerthan 270 MPa. Sample No. 27 in which the amount of Nb is small has thenvalue of less than 0.20 and the excessively low r value.

Sample No. 22 as the conventional ultra low carbon high strength coldrolled steel sheet has the YS of much larger than 270 MPa, and the nvalue of less than 0.20.

In each of samples Nos. 1 to 19, the ferrite grains are fine having adiameter of less than 10 μm as compared with that of sample No. 22 ofthe conventional example, 11.4 μm. Therefore, each of samples Nos. 1 to19 has the superior resistance to the occurrence of the orange peel andthe anti-secondary work embrittlement.

TABLE 1 (mass %) STEEL C Si Mn P S sol. Al N Nb B OTHERS Nb/C REMARKS A0.0065 0.17 1.7 0.052 0.003 0.12 0.0026 0.095 — — 1.9 WITHIN THE PRESENTINVENTION B 0.0067 0.17 1.6 0.050 0.005 0.28 0.0023 0.101 — — 1.9 WITHINTHE PRESENT INVENTION C 0.0064 0.13 1.6 0.037 0.002 0.75 0.0022 0.103 —— 2.1 WITHIN THE PRESENT INVENTION D 0.0064 0.10 1.6 0.022 0.002 1.200.0014 0.098 — — 2.0 WITHIN THE PRESENT INVENTION E 0.0043  0.003  0.140.013 0.001 0.21 0.0026 0.075 — — 2.3 WITHIN THE PRESENT INVENTION F0.0055 0.05  0.85 0.045 0.004 0.21 0.0026 0.075 — — 1.8 WITHIN THEPRESENT INVENTION G 0.0097 0.06 1.9 0.035 0.003 0.75 0.0025 0.130 — —1.7 WITHIN THE PRESENT INVENTION H 0.0040 0.25 1.2 0.068 0.006 0.350.0016 0.043 — — 1.4 WITHIN THE PRESENT INVENTION I 0.0155 0.10 0.60.057 0.004 0.34 0.0034 0.162 — — 1.3 WITHIN THE PRESENT INVENTION J0.0052 0.25 1.6 0.041 0.004 0.52 0.0034 0.081 0.0002 — 2.0 WITHIN THEPRESENT INVENTION K 0.0055 0.25 1.6 0.042 0.005 0.51 0.0024 0.094 0.0018— 2.2 WITHIN THE PRESENT INVENTION L 0.0068 0.18 1.4 0.051 0.005 0.300.0021 0.102 0.0004 Cu: 0.2, 1.9 WITHIN THE PRESENT INVENTION Ni: 0.2 M0.0080 0.18 1.3 0.047 0.001 0.30 0.0022 0.099 0.0003 Cr: 0.2, 1.6 WITHINTHE PRESENT INVENTION Mo: 0.3, Ti: 0.01 N 0.0077 0.18 1.7 0.050 0.0050.30 0.0037 0.103 0.0004 Sb: 0.01, 1.7 WITHIN THE PRESENT INVENTION Sn:0.003 O 0.0067 0.25 1.9 0.042 0.005  0.045 0.0029 0.101 — — 1.9 OUT OFTHE PRESENT INVENTION P 0.0067 0.01 1.9 0.005 0.005 1.62 0.0028 0.105 —— 2.0 OUT OF THE PRESENT INVENTION Q 0.0018 0.25 2.4 0.044 0.008 0.030.0025 0.024 — — 1.7 OUT OF THE PRESENT INVENTION R 0.0250 0.10 1.80.040 0.006 0.23 0.0025 0.200 — — 1.0 OUT OF THE PRESENT INVENTION S0.0055 1.70 0.3 0.005 0.002 0.15 0.0025 0.070 — — 1.6 OUT OF THE PRESENTINVENTION T 0.0050 0.01 3.5 0.010 0.004 0.18 0.0022 0.070 — — 1.8 OUT OFTHE PRESENT INVENTION U 0.0056 0.01 0.7 0.160 0.001 0.19 0.0024 0.065 —— 1.5 OUT OF THE PRESENT INVENTION V 0.0045 0.15 1.7 0.060 0.004 0.250.0020 0.024 — — 0.7 OUT OF THE PRESENT INVENTION UNDERLINED: OUT OF THEPRESENT INVENTION

TABLE 2 HOT ANNEAL- AREA {770 + ROLLING ING DENSITY ([sol. Al] − CONDI-CONDI- MECHANICAL GRAIN WIDTH OF Nb (C, N) 0.085)^(0.24) × TIONS TIONSPROPERTIES DIA- OF OF 50 nm SAMPLE STEEL 820} SRT CT AT YS r TS METERPFZ OR MORE NO. NO. (° C.) ※ (° C.) (° C.) (° C.) LINE (Mpa) n₁₋₁₀ VALUE(Mpa) (μm) (μm) (/μm²) REMARKS 1 A 1,137 1,100 560 830 CGL 269 0.2021.81 442 7.2 0.35 0.049 EXAMPLE 2 B 1,324 1,090 560 830 CGL 253 0.2161.88 441 7.5 0.55 0.000 EXAMPLE 3 1,324 1,230 560 830 CGL 257 0.212 1.86442 7.3 0.58 0.005 EXAMPLE 4 1,324 1,280 560 830 CGL 259 0.211 1.86 4437.1 0.50 0.020 EXAMPLE 5 1,324 1,230 490 865 CGL 255 0.215 1.83 447 6.30.60 0.000 EXAMPLE 6 1,324 1,230 495 865 CAL 257 0.213 1.98 446 6.6 0.750.006 EXAMPLE 7 C 1,350 1,230 560 830 CGL 264 0.207 1.96 444 7.3 0.440.029 EXAMPLE 8 D 1,350 1,220 560 830 CGL 269 0.203 1.94 442 7.4 0.390.030 EXAMPLE 9 E 1,268 1,220 620 865 CGL 169 0.219 1.90 340 8.0 1.300.012 EXAMPLE 10 F 1,268 1,230 580 855 CGL 205 0.217 1.87 396 7.8 0.500.010 EXAMPLE 11 1,268 1,230 500 720 BAF 198 0.219 1.91 397 6.5 0.450.006 EXAMPLE 12 G 1,350 1,200 500 865 CGL 262 0.211 1.93 451 6.4 0.250.040 EXAMPLE 13 H 1,350 1,220 525 800 CAL 263 0.202 1.86 446 8.1 0.370.008 EXAMPLE 14 I 1,350 1,230 560 830 CGL 269 0.200 1.90 441 6.2 0.320.010 EXAMPLE 15 J 1,350 1,230 570 850 CAL 258 0.209 2.05 444 6.9 0.520.027 EXAMPLE 16 K 1,350 1,220 580 855 CAL 259 0.210 2.11 446 6.7 0.410.014 EXAMPLE 17 L 1,337 1,250 580 850 CGL 254 0.214 1.97 444 7.4 0.490.000 EXAMPLE 18 M 1,337 1,250 610 850 CGL 265 0.208 1.94 448 6.5 0.380.000 EXAMPLE 19 N 1,337 1,220 580 855 CGL 259 0.210 1.90 446 7.0 0.420.008 EXAMPLE 20 O — 1,220 560 830 CGL 279 0.193 1.73 445 7.4 0.22 0.116COM- PARATIVE 21 P 1,350 1,230 560 830 CGL 276 0.192 1.93 444 7.5 0  0.045 COM- PARATIVE 22 Q — 1,230 620 830 CGL 294 0.181 1.57 443 11.4 0   0.010 CONVEN- TIONAL 23 R 1,286 1,220 590 860 CGL 314 0.190 1.62 4726.3 0   0.064 COM- PARATIVE 24 S 1,196 1,220 560 830 CGL 302 0.193 1.78485 7.8 0.05 0.042 COM- PARATIVE 25 T 1,236 1,220 560 820 CGL 353 0.1321.92 444 4.8 0   0.055 COM- PARATIVE 26 U 1,247 1,220 560 830 CGL 3110.193 1.79 482 7.6 0.08 0.053 COM- PARATIVE 27 V 1,302 1,230 560 830 CGL320 0.160 1.25 442 10.0  0   0.012 COM- PARATIVE UNDERLINED: OUT OF THEPRESENT INVENTION ※A VALUE EXCEEDING 1350° C. IS REGARDED AS 1350° C.

1. A high strength cold rolled steel sheet consisting of 0.004 to 0.02%of C, 1.5% or less of Si, 3% or less of Mn, 0.15% or less of P, 0.02% orless of S, 0.001 to 0.007% of N, 0.03 to 0.2% of Nb, 0.2 to 0.6% ofsol.Al by mass, and the balance of Fe and inevitable impurities andcomposed of ferrite grains having an average grain diameter of 10 μm orless, in which the average number per unit area (hereinafter referred toas “average area density”) of Nb(C, N) precipitates having a diameter of50 nm or more in the ferrite grains is 7.0×10⁻²/μm² or less, and a zonehaving a width of 0.2 to 2.4 μm and an average area density of NbCprecipitates of 60% or less of that of the central portion of theferrite grains is formed along grain boundaries of the ferrite grains.2. The high strength cold roiled steel sheet according to claim 1,wherein the following equation (1) is satisfied;([Nb]/[C])×(12/93)≧1  (1), where [Nb] and [C] represent the amounts ofNb and C (mass %), respectively.
 3. The high strength cold roiled steelsheet according to claim 1, further containing 0.0001 to 0.003% of B. 4.The high strength cold rolled steel sheet according to claim 2 furthercontaining 0.0001 to 0.003% of B.
 5. The high strength cold rolled steelsheet according to claim 1 further containing at least one elementselected from the group consisting of 0.5% or less of Cu, 0.5% or lessof Ni, 0.3% or less of Mo, 0.5% or less of Cr, and 0.04% or less of Ti.6. The high strength cold roiled steel sheet according to claim 1further containing at least one element selected from the groupconsisting of 0.5% or less of Cu, 0.5% or less of Ni, 0.3% or less ofMo, 0.5% or less of Cr, and 0.04% or less of Ti.
 7. The high strengthcold rolled steel sheet according to claim 1 further containing at leastone element selected front the group consisting of 0.2% or less of Sband 0.2% or less of Sn, wherein the following equation (2) is satisfied;0.002≦[Sb]+½×[Sn]≦0.2  (2), where [Sb] and [Sn] represent the amounts ofSb and Sn (mass %), respectively.
 8. The high strength cold rolled steelsheet according to claim 6 further containing at least one elementselected from the group consisting of 0.2% or less of Sb and 0.2% orless of Sn, wherein the following equation (2) is satisfied;0.002≦[Sb]+½×[Sn]≦0.2  (2), where [Sb] and [Sn] represent the amounts ofSb and Sn (mass %), respectively.
 9. A method for manufacturing a highstrength cold rolled steel sheet comprising the steps of: hot rolling asteel slab having the chemical composition according to claim 1 into ahot rolled steel sheet after heating the steel slab at a heatingtemperature SRT which satisfies the following equations (3) and (4); andpickling and cold rolling the hot rolled steel sheet, followed, byannealing within a temperature range of a ferrite phase above therecrystallization temperature,SRT≦1350° C.  (3)1050° C.≦SRT≦{770+([sol.Al]−0.085)^(0.24)×820}° C.  (4), where [sol.Al]represents the amount of sol.Al (mass %).
 10. The high strength coldrolled steel sheet according to claim 1 having an n₁₋₁₀ of 0.20 or more.11. The high strength cold rolled steel sheet according to claim 1having a YS of 270 MPa or less.
 12. The high strength cold rolled steelsheet of claim 1 having a TS of 340 MPa or more.